Unbalanced Line

An unbalanced line is a type of electrical interconnect in which there is only one signal conductor, or a line with two signal conductors where the impedance to ground from each line is not equal. Unbalanced lines are susceptible to radio frequency interference as well as other types of noise, such as ground noise.

See these two articles for a full explanation:

http://www.jensentransformers.com/an/an002.pdf
http://www.jensentransformers.com/an/an003.pdf

Both articles come from Bill Whitlock at Jensen Transformers, and are accessible here:
http://www.jensentransformers.com/apps_wp.html